Commit b36637c8f7 set
`__ETC_PROFILE_NIX_SOURCED` globally, but this is not enough to prevent
the script from being run again by child shells, because the
variable was not exported and thus not inherited by any child process.
Exporting the variable also agrees with the bash scripts.
Notably, the old behavior broke `nix develop -c fish` in some cases,
because the profile bin directory got prepended to the path, causing
binaries from the profile to override binareis from the devshell.
b9ed3ae36e